Extensible and feature rich, written in Python. Powerline was
completely rewritten in Python to get rid of as much vimscript as
possible. This has allowed much better extensibility, leaner and better
config files, and a structured, object-oriented codebase with no mandatory
third-party dependencies other than a Python interpreter.

Stable and testable code base. Using Python has allowed unit testing
of all the project code. The code is tested to work in Python 2.6+ and
Python 3.

Support for prompts and statuslines in many applications. Originally
created exclusively for vim statuslines, the project has evolved to
provide statuslines in tmux and several WMs, and prompts for shells like
bash/zsh and other applications. It’s simple to write renderers for any
other applications that Powerline doesn’t yet support.

Configuration and colorschemes written in JSON. JSON is
a standardized, simple and easy to use file format that allows for easy
user configuration across all of Powerline’s supported applications.

Fast and lightweight, with daemon support for even better performance.
Although the code base spans a couple of thousand lines of code with no
goal of “less than X lines of code”, the main focus is on good performance
and as little code as possible while still providing a rich set of
features. The new daemon also ensures that only one Python instance is
launched for prompts and statuslines, which provides excellent
performance.

But I hate Python / I don’t need shell prompts / this is just too much
hassle for me / what happened to the original vim-powerline project / …

You should check out some of the Powerline derivatives. The most lightweight
and feature-rich alternative is currently Bailey Ling’s vim-airline project.